Conventional microwave mechanical, electro-mechanical, and electronic switches may not compatible with on-chip integration with, and cryogenic operation of, superconducting electronic circuits, because of incompatible fabrication processes and high power dissipation. Likewise, tunable filters that are commonly realized by use of either active components such as voltage-variable capacitors (i.e., varactors), mechanical drivers, or ferroelectric and ferrite materials, are not easily controllable by signal levels that can be generated with single flux quantum (SFQ) technologies, and many are not operable at cryogenic temperatures. While superconducting microwave filters, both fixed and tunable, have been previously realized using both high temperature and low temperature superconductors, their use in switching applications suffers from high return loss, limited usable bandwidth, and poor out-of-band off-state isolation.
In certain superconducting contexts, a coupler can be provided to exchange information between objects by turning on some coupling between them, or to isolate the objects by turning off that coupling. A tunable coupler is one that controls a degree of signal coupling between two objects, i.e., between pure “on” (coupled) and pure “off” (uncoupled) states, by the provision of one or more variable control signals.